1. Field of the Invention
This invention relates to constant velocity universal joints and, more particularly, to a method for making the cage member used therein.
2. Description of the Prior Art
The present invention is a cage for guiding balls used in constant velocity universal joints. The construction and operation of constant velocity joints, and the use of such cages, can be understood as follows.
It is well known in the art that torque may be transmitted between two shafts which articulate relative to each other through use of a constant velocity joint. Such joints transmit torque through balls which are arranged between an inner and an outer joint member, each having guide grooves for the balls. In order to obtain constant velocity in every angular position of the joint during articulation, the balls must be guided in such a way that at each bending angle of the joint the balls are arranged in a so called constant velocity plane, that is, a plane which passes through the instantaneous center of the joint and bisects the instantaneous input and output axes of the joint. Such axes are the rotational axes of the two joint members and the joint center is the point at which these axes intersect.
For keeping the balls always in the same plane, it is a known practice to keep the balls not only in guide grooves, but also in windows of a cage member which is arranged between the two joint members. For positively guiding the balls in a constant velocity plane during articulation of the joint, the cage member is guided within the outer joint member by a spherical surface on the outer surface of the cage and is guided with respect to the inner joint member by a spherical surface on the inner surface of the cage. A known form of constant velocity joint of this type is described in U.S. Pat. No. 2,046,584 to Rzeppa.
A typical constant velocity joint is shown in FIG. 1, where the joint 10 is structured to transmit torque between two mutually articulating shafts 12 and 14. This is accomplished by use of an annular outer member 16 having grooves 18 on its inner surface 20 parallel with the axis 22 of the joint. An inner joint member 24 is positioned within the outer joint member and is provided with grooves 26 on its outer surface 28 to form pairs of opposed grooves with the grooves on the outer joint member. A ball 30 is positioned in every pair of opposed grooves and a ball restraining member or cage 32 disposed between the two joint members retains the balls in a plane perpendicular to the axis of the cage when the joints are in the neutral position. The cage, as can be seen by reference to FIG. 2, is constructed in one piece of annular spherical shape, having spaced apart windows 34 in which the balls are guided. Guidance for the balls, in order to assure constant velocity characteristics for the joint, is provided by contact between the ball and the window sides 36. The cage is guided within the outer joint member by a spherical surface 38 on the outer surface of the cage and is guided with respect to the inner joint member by a spherical surface 40 on the inner surface of the cage. The centers of the inner and outer spherical surfaces of the cage are equidistant on opposite sides of the plane passing through the centers of the balls.
As a result of the inner and outer spherical surfaces of the cage being equidistant on opposite sides of the plane passing through the center of the balls, during joint articulation through a predetermined angle, the inner joint member, in effect, pivots around the offset center of the inner surface of the cage while the outer joint member pivots around the offset center of the outer surface of the cage. As the cage is constrained within the outer joint member, the center of the outer joint member surface must lie on the joint axis. In addition, the equal offset of the centers of the inner and outer spherical surfaces of the cage ensures that the plane of the ball centers articulates accurately through one half of a predetermined joint angle, thus, maintaining constant velocity characteristics. During articulation through a predetermined joint angle, the center of the inner surface of the cage moves off the joint axis, resulting in the establishment of a new ball center plane displaced slightly from the theoretical joint plane center, i e., the center at zero joint angle, resulting in a very small axial displacement of the joint center thereby requiring some axial movement of the cage with respect to the outer joint member. Accordingly, there is no tendency to jam and the geometry is such that the cage inner and outer surface centers always remain equally offset from the instantaneous joint center which passes through the center of the balls. That is, the plane of the ball centers is always the true median plane of the joint.
FIG. 2 shows a prior art constant velocity joint cage of the type used to retain the torque transmitting balls used in constant velocity joints. As can be seen, the cage 32 is formed of one piece in the shape of an annular section of a sphere truncated approximately symmetrically about the window centerline 42. Both the spherical inner surface 40 and the spherical outer surface 38 of the cage may be positioned around offset centerlines. This is particularly shown in FIG. 3 where the spherical inner surface 40 and spherical outer surface 38 of the cage 32 have slightly off-set centerlines 44 and 46, respectively; and where the inner surface diameter 48 is less than the outer surface diameter 50. The windows 34, as shown in FIG. 2, are usually six in number matching the number of grooves in each of the inner and outer joint members. The windows are positioned symmetrically about the cage and may be of various shapes and have a particular window centerline 42 as required for a particular constant velocity joint application.
The critical tolerances are the window width 54 and the relationship between the centerline of the windows and the centerlines of the inner and outer spherical surfaces. These are critical for proper functioning and operational life-time of the constant velocity joint. The width of the window is critical because window sides parallel to the window centerline 42 serve as the ball guide surfaces or sides 36 which make contact with the ball, guiding it in a direction transverse to the window centerline, and as the shafts articulate it is desirable to minimize play therebetween. The window ends 56 need only be far enough apart so as to ensure that the balls will not be interfered with, as the grooves 26 in the inner joint member 24 and the grooves 18 in the outer joint member 16 serve to guide the balls in a direction transverse to the window centerline. This is a particularly important fact for the method of making constant velocity joint cages according to the present invention, as will be seen, following.
It is apparent from the foregoing that the ball retaining cage must be manufactured to close tolerances in order to assure that the balls will always be guided, as the joint articulates, in the true median plane of the joint. Of particular sensitivity is the shape of the cage and the dimensioning of the windows.
The current practice in the art is to make the cages as follows:
A segment is cut from a length of tubing. The segment is then formed into an annular spherical shape followed by rough finishing by turning them to approximate size and shape. Thereafter, windows are punched or cut in the segment followed by heat treating and finish grinding. This process is a natural outgrowth of the shape and function of the part and the generally available methods to produce it. The method currently in use results in high processing and excess offal costs, as well as high scrap, and rework rates.
The present invention is an improved method by which the ball retaining cage used in constant velocity joints may be manufactured simply, easily, and accurately, at a lower cost with a minimum of offal and scrap.